Thermal inertia lamp cathode testing



Dec. 29, 1953 R. N. THAYER 2,664,543

THERMAL INERTIA LAMP CATHODE TESTING I Filed Aug. 9, 1950 Fig.1.

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Pa tentecl Dec. 29, 1953 THERMAL INERTIA LAMP CATHODE TESTING Richard N. Thayer, Cleveland Heights, Ohio, assignor to General Electric Company, a. corporation of New York Application August 9, 1950, Serial No. 178,526

8 Claims.

This invention relates generally to the manufacturing of electric discharge lamps having cathodes of the activated type, and more particularly to a method of measuring the quantity of emission mixture deposited on the cathode.

It is well known, of course, that the performance and life of electric discharge devices, in particular, discharge lamps of the ffuoroescent type, are very much influenced by the amount of activated emission mix deposited on the cathodes. The cathodes are generally constructed of tungsten wire formed into a coil of suitable dimensions and are mounted on stem presses at opposite ends of thelamps. The tungsten itself does not have sufiicient electron emission to carry effectively the arc discharge current of the lamp during operation. Accordingly, the cathodes are provided with some activating substance which emits electrons more readily than the tungsten itself. Such an activating substance maybe a mixture of barium and strontium oxides. When heated, barium or strontium oxides emit electrons copiously and support the are discharge current without sputtering and with only a small heat loss.

It is essential to the successful manufacture of lamps that the amount of emission mixture on the filamentary cathodes be accurately controlled in order to assure uniform quality in the lamps. In the past, the amount of emission mixture on the cathodes of a lamp was generally determined by fracturing the lamp, removing the cathode, weighing it and subsequently dissolving out the emission mixture by means of acids and again weighing the cathode. The difference between the two weights gave the amount of emission mixture dissolved by the acid and, accordingly, the desired figure. It need hardly be said that such a method of weighing or measuring the amount of emission mixture on a cathode is wasteful since the lamp is destroyed in the process. At best it can only serve as a check for sampling lamps out of a production batch.

Accordingly, it is an object of my invention'to provide a new and improved nondestructive method of measuring the amount of emission mixture on a, filamentary cathode for discharge device.

A further object of my invention is to provide an improved method, suitable for production line usage, of measuring the amount of emission mixture on the filamentary cathodes of an electric discharge device which method does not cause any deterioration or modification of the operating characteristics of the device.

A further object of my invention is to provide apparatus for readily carrying out my method of testing.

My improved testing method utilizes basically a Wheatstone bridge circuit in a manner to compare the rate of heating of a cathode coated with a quantity of the emission mixture which is to be determined with the rate of heating of an uncoated cathode. The method is applicable to any type of electrode wherein the conductive element or coil has an appreciable temperature coefiicient of resistance. For instance, the usual tungsten filament cathodes of fluoroescent lamps have a high positive temperature coefiicient of resistance. However, the same method could be applied to electrode elements having a lesser positive coefiicient, or even having a negative coeiiicient, just so long as the coefficient is not zero. The diiference in temperature which develops during the course of heating momentarily causes a difference in the electrical resistances of the two cathodes which,.in turn, causes an unbalance in the bridge. This unbalance may be measured through a suitable indicating device and, by comparison with suitable calibration charts, gives an indication of the amount of emission mixture on the coated cathode.

For further objects and advantages and for a better understanding of the invention, attention is now directed to the following description and accompanying drawings. The features of the invention believed to be novel will be more particularly pointed out in the appended claims.

In the drawings:

Fig. l is a schematic diagram showing the essential elements of a Wheatstone bridge circuit for carrying out the method of my invention.

Fig. 2 is a schematic diagram of an improved detecting or indicating system incorporating a vacuum tube voltmeter which may be utilized as an indicator with the bridge circuit of Fig. 1.

In carrying out the method of my invention, aWheatstone bridge of the usual type may be utilized such as is illustrated in Fig. l. 'ihe two pairs of conjugate points of the bridge are denoted by A-B, and 0-D. A source of voltage, for instance, a commercial supply at 110 volts, cycles, is connected in series with a current limiting resistor 1 across injut conjugate points AB. The electrode terminals of the lamp wherein the amount of emission mixture deposited upon the electrode is to be determined are connected as the unknown impedance between points A-C. A similar lamp of standard construction but wherein the usual emission mixnected between the output conjugate points pr terminals CD. s p

In carrying out the preferred form oimy method of determining the amount of emission mix on a lamp cathode, the terminals ofth unknown coated lamp and the terminals bf astandard uncoated lamp are connected as shown in the circuit of Fig. 1. An alternating voltage is applied to the bridge sufiicint'td causeacur 4 a number of lamps. Afterwards, the weight of emission material on these same lamps is determined experimentally by removing the cathodes and weighing the elements directly in the manner previously described. When a tabulation of sufficient scope has been obtained, a calibration chart 'may be 'drawn up and the amount of emission mix on'any electrode structure of the same type determined simply by reading the maximum deflection on the indicator and comparing it to the curve on the chart.

'Themethod described in the foregoing para- "graphs constitutes the preferred formof my inrent to flow through the lamp electro'des-ofa value approximately equal to that utilized by the lamp'innormal operation. After fio'seconds, orslightlyless, when both-lamps have reached equilibrium temperature, the bridge is balanced by adjus ment o ,r sist efi s that deflectio he indi a r is d e o zero. n the voltage source is disconnected from the bridge for a time sufficient to allow the cathodes to cool to approximately ambient temperature. Eorthe usual commercial fluorescent lamps, a waiting period of approximately 30 to 60 seconds e e a y uf ic A Thevoltage supply is, then reconnected across the bridge terminals and the maximum deflection of the indicator i is noted during the time interval immediately following. The resistances of the tungsten coils of the cathodes may increase over'a ratio of as much as 10 to 1 from ambient to equilibrium temperature, due to the positive temperature coeihcient of tungsten. Since the bridge was previously balanced with both electrodes hot and at approximately the same temperature and since both electrodes are now starting at ambient temperature, their resistances are originally approximately equal and the bridge is approximately balanced; It will be understood that after a sufficient time interval, both electrodes will reach the previously mentioned equilibrium temperature and the bridge will again be, balanced. However, during the course of warming up, the coated electrode of lamp 2, due to. its, greater thermal mass or inertia, will lag behind the uncoated electrode of lamp 3 in reaching equilibrium temperature; and since it is cooler, its resistance at any instant will, be comparatively less. In other words, during the warming-up process, the, resistances of the electrodes of lamps 2 and 3 attaina maximum instantaneous deviation. Accordingly, the indicator starts off near balance, reaches a maxideflection or unbalance, and eventually returns to balance. The deflection of theindicator is noted at maximum unbalance. The maximum deflection of the indicator during the warming-up process is a function of the mass of emission material on the unknown lamp as compared with the known standard lamp.

It is. possible to compute directly, by means of mathematical calculations, the weight of emissionmix on the electrode of lamp 2 knowing the values of voltage applied to the circuit and the specificthermal capacity of the electrode'structures-of the-lamps. In practice, however, it is more convenient to determine experimentally the significance of they maximum deflection indications by obtaining a'range of such deflections for vention. However, it is possible to utilize a simplified procedure, although with some sacrifice in accuracy. According to this simplified procedure, the bridge is balanced with the electrodes cold, and then the maximum momentary unbalance is noted during the course of heating the cathodes by means of a predetermined higher voltage, to a higher temperature. Since both electrodes have approximately the same temperature coefficient of resistance, they would be approximatly balanced at equilibrium at the higher temperature so that approximately the same results are achieved.

Under the usual conditions of manufacturing and production testing, it is highly desirable to have a compact testing equipment by means of which the method may be readily carried out. An indicating unit which may be utilized in conjunction with the bridge circuit of Fig. l-is illustrated schematically in Fig. 2. This unit; is, essentially, a vacuum tube voltmeter of fairly simple design and comprising a transformer 10 whereof the terminals of the primary winding II are connectedacross output conjugate, points CD of the bridge. The secondary winding l2 has a center tap which is grounded and has its outside terminals connected to the anodes of a full wave rectifying tube 43. The cathode of tube I3 is connected to develop a positive unidirectional voltage across a capacitor l4 and a resistor 15 connected in parallel through a limiting resistance 16. This arrangementwill be recognized as a full Wave detecting system of conventional form, and the purpose of the transformer is simply to convert the bridge impedance to that required for eificient operation of the detecting system. The voltage supplied by the detector tube is provided through a second current limiting resistor i! to the control grid of one of a pair of pentode tubes 18 and I9. These tubes are connected in similar circuits in parallel across a source of unidirectional voltage indicated byB-iand 13-. A micro-ammeter 20 is connected, in series with a sensitivity adjusting resistor 2!, across the anodes of the two tubes, and voltage is supplied to the tubes from an adjustable balancing potentiometer 22. v The cathodes of the tubes are connected to a source of voltage slightly more negative than ground. as indicated at B+. The control grid of tube I8 is connected to detector 13 as has previously been mentioned, whereas the control grid of tube [9 is connected to ground through a resistor 23. When the bridge is balanced or disconnected from the alternating voltage supply, no voltage appears across terminals C- -D; and, -'accordingly, both tubes 18 and l9'conduct equally so that the deflection on meter "20 is zero. "As soon as a voltage appearsacross points CDof the bridge, detector-i3 causes a positive'unidirectional voltage to appearat the-control'grid tube l8, which increases'the conduction through tube l8 and upsets the voltage equilibrium at the" anodes of tubes I 8 and [9, respectively. Meter 20 thereupon indicates this condition Of -unbalance which, in turn, is a function of the alternating voltage existing between points C and D and of the unbalance in the bridge.

While my method of determining the amount of emission mix on an electrode has been described involving a comparison with an electrode of a standard lamp which is completely uncoated, it will, of course, be understood that various modifications may be made without departing from the invention. Thus, the electrode of the standard lamp may likewise be coated, or coated to a lesser degree, in which case a comparison may likewise be obtained providing the amount of emission mixture on the unknown lamp is always either greater or lesser than the amount on the standard lamp. However, even where the amounts may vary in either direction, the

method may, nevertheless, be carried out .by;

utilizing an indicating system which is sensitive to the direction of unbalance between the points C and D. The appended claims are accordingly intended to cover any such modifications coming Within the tru spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The method of determining a figure characteristic of the quantity of emission material on a resistive electrode for a discharge device by means of a Wheatstone bridge apparatus, said electrode having an appreciable temperature coefiicient of resistance, which comprises the steps of balancing the bridge with said electrode in the unknown arm and a similar electrode having a known thermal inertia in the standard arm, energizing the bridge to cause said electrodes to tend towards a new thermal equilibrium, and measuring the maximum temporary unbalance of said bridge during the course of attaining said new equilibrium.

2. The method of determining a figure characteristic of the quantity of emission material on a resistive electrode for a discharge device by means of a Wheatstone bridge apparatus, said electrode having an appreciable temperature coefiicient of resistance, which comprises the steps of balancing said bridge with said electrode in the unknown arm thereof and a similar electrode coated with a known quantity of emission material in the known arm, said balance being achieved with said electrodes near ambient temperature, increasing the energization of the bridge to cause said electrodes to tend towards a higher thermal equilibrium, and measuring the maximum temporary unbalance in said bridge during the course of attaining said higher equilibrium.

3. The method of determining a figure characteristic of the quantity of emission material on a resistive cathode for a discharge device by means or" a Wheatstone bridge apparatus, said electrode having an appreciable temperature coeificient of resistance, which comprises the steps of balancing the bridge with said cathode in the unknown arm and a similar cathode coated with a known weight of emission material in the standard arm, said balance being made with both cathodes at a relatively high thermal equilibrium, deenergizing the bridge apparatus and allowing said cathodes to cool to approximately ambient temperature, and finally reenergizing the bridge apparatus and measuring the maximum unbalance thereof during the course of 6. reheating said cathodes" to said equilibrium temperature.

' 4;. The method of determining a figure characteristic of the quantity of emission material on a discharge lamp resistive cathode having an appreciable temperature coeificient of resistance, by means of a Wheatstone bridge apparatus, which comprises the steps of balancing the bridge with said cathode. in the unknown arm and an uncoated cathode of similar construction in the standard arm, said balance being made with both said cathodes at a relatively high thermal equilibrium temperature, de-energizing the bridge apparatus and allowing both said cathodes to cool to approximately ambient temperature, and finally measuring the maximum unbalance of said bridge, after re-energization thereof, during the course of heating said cathodes to said equilibrium temperature.

5. Th method of determining a. figure characteristic of the weight of emission material on,

a cathode for electric discharge lamps by means of a Wheatstone bridge apparatus comprising an unknown arm, a standard arm, a pair of ratio arms, and an indicator, said cathode having an appreciable temperature coefficient of resistance, which comprises the steps of balancing said bridge with said cathode connected in the unknown arm and an uncoated cathode of similar construction in the standard arm, said balance being achieved with both said cathodes at a relatively high thermal equilibrium, de-energizing said bridge and allowing both said cathodes to cool to approximately ambient temperature, reenergizing said bridge and noting the maximum unbalance thereof during the course of reheating said cathodes to said high thermal equilibrium temperature.

6. The method of determining a figure characteristic of the weight of emission material on a resistive cathode having a positive temperature coefiicient of resistance, by means of a Wheatstone bridge apparatus of the type including a pair of ratio arms, an unknown arm, a standard arm, and an indicator, which comprises the steps of energizing and balancing said bridge with said cathode inthe unknown arm and an uncoated cathode of similar construction in the known arm, said balancing being achieved at a relatively high thermal equilibrium, tie-energ zing said bridge and allowing said cathodes to cool to near ambient temperature, re-energizing said bridge, and measuring the maximum unbalance thereor during the course of reheating said cathodes towaru said high equilibrium temperature.

7. The method or determining a figure characteristic or the weight or emission material on a cathode of the filamentary thermionic type having a positive temperature coemcient or resistance, by means of a Wheatstone bridge apparatus of the type including a pair of ratio arms, an unknown arm, a standard arm, and an indicator, which comprises the steps or energizing and balancing said bridge with said cathode in the unknown arm and an uncoated cathode of similar construction in the known arm, said balancing being achieved at a relatively high thermal equilibrium where the resistances of said cathodes are many times greater than their resistances at ambient temperature, de-energizing said bridge and allowing said cathodes to cool to near ambient temperature, re-energizing said bridge, and measuring the maximum unbalance thereof during the course of reheating said cathodes toward said high equilibrium temperature.

.8; a The method of determining -a, figure charace,v

a filamentary thermionic cathoderor. .an -electric discharge device, said cathode compr-ising tunge sten wire having a high positiyetemperature co efiicient of resistance, by means of, a =Wheatstone:

bridge apparatus of thetypeincluding :a; pair of, ratio arms, an unknownrarm; astandard .arm. and .an indicator, which methodcomprises the steps of energizing and balancing said bridge with said cathode in the unknown arm and anuncoated cathode of similar construction in theistandard arm, said balancing. beingwachieved with. said cathodes at a relatively high thermal equilibrium where the resistances of said cathodes are many times-greater than their. resistances atambient temperature, die-energizingsaid bridge and .a1 lowing said cathodes to cool to near ambientltemperature, re-energizing said bridge, and measuring the-maximumdeflection in said-'indicatoi as said cathodes increase in temperature toward said high thermal,"equilibrium,v said defiection being caused by unbalance in said bridge due to the thermal lag; of :said coated cathode over said unccated cathode causing its resistance to momentarily increase at a lesser rate.

RICHARD N. THAYER..

References .Citedin the fileof this patent UNITED STATES PATENTS Number Name Date 1,695,424 Harrison Dec. 18, 1928 2,315,593 Cassen Apr 6, 1943 OTHER REFERENCES Measuring Resistances of Hot Filaments, by

A.= K. McLaren, Radio, March 1945.

(Copy in Div. 48;) 

